In the glass manufacturing process glass sheets (known as float glass) can be combined with different kind of elements, such as coated or tempered layers to create glass panes for different purposes having specific properties. For example, insulating glass units, IGUs, are constructed typically with a configuration having two or more glass sheets with a closed spacing in between the sheets, where the closed spacing is filled with gas with low thermal conductivity, such as Oxygen, Argon, Xenon, Krypton Nitrogen or mixture of those.
In addition to gases with low thermal conductivity, also low emissivity coatings are used for a considerable reduction of heat transfer in window glazing units. Most of the commercial coatings have metallic substances in them and are thus electrically conductive. They reflect also visible light better than the bare glass surface. Since the low emissivity coating is typically inside an insulating glass unit for mechanical and chemical protection, it cannot be directly accessed for conductivity measurement.
When glass panes are manufactured, their quality must be evaluated according to the manufacturer's quality system, standards or local regulations. The most typical parameters of a glass pane are its mechanical dimensions, thickness of the glass panes, thickness of the possible lamination(s) or coating(s), spacing between the panes, locations of the possible surface coatings and possible toughening or tempering of the glass panes, but also the concentration of the gas component(s) in the spacing.
Different kinds of solutions are known from the prior art for determining the concentration of gas components in a gas mixture contained in the spacing, such as taking a gas sample from the spacing and measuring it e.g. by using a mass spectrometer. Taking the gas sample is invasive method, whereupon the surface of the glass unit must be broken, which is clear disadvantage of the invasive methods. Also non-invasive methods are known, such as discharging the spacing between the panels of the glass unit by applying rapidly alternating electrical field to the spacing, and analyzing the emitted discharge light. The discharge can be created by a needle-like electrode, whereupon the inner coating layer of the glass unit made of conducting material is used as another electrode.
There are also some drawbacks with the known non-invasive methods. For example if the glass pane is coated or laminated with plastic film, another electrode must be provided in another way. In addition when at least one sheet has a metallically coated surface, the measurement cannot be done through the metallic surface, since the rapidly alternating electrical field cannot penetrate the metallic surface. Furthermore only one spacing can be measured at a time.